Conventionally when a one-dimensional solid-state imager is used to observe an object that moves at a high speed, there is a problem in that the signal-to-noise ratio of an output signal is reduced. This is because the storage time of signal charges at the light-to-electricity conversion part is too short and the signal charge amount corresponding to the observed image is too small. In order to solve this problem, it is proposed to enhance the signal-to-noise ratio of the output signal by adding signal charges corresponding to the same observed image to signal charges produced by observed images that move on the light-to-electricity conversion part. Such operation is called T.D.I. (Time-Delay Integration) operation and this operation is effective only in the one-dimensional solid-state imager. A solid-state imager of T.D.I. operation type will be described with reference to the drawings.
FIG. 3 shows a construction of a prior art one-dimensional T.D.I. type solid-state imager. In FIG. 3, a light-to-electricity conversion part 1 is provided for storing signal charges which are generated in accordance with the light incident to the imager. A vertical charge transfer part (hereinafter referred to as vertical CCD) constituted by a charge coupled device (hereinafter referred as CCD) is provided for transferring charges generated at the light-to-electricity conversion part 1 in the vertical direction. A horizontal charge transfer part 3 (hereinafter referred to as a horizontal CCD) is provided for receiving signal charges transferred by the vertical CCD 2 and transferring the same in the horizontal direction. A transfer gate 4 is provided between the light-to-electricity conversion part 1 and the vertical CCD 2 for controlling the transfer of the signal charges at the light-to-electricity conversion part 1 to the vertical CCD 2. An output part 5 is provided for outputting the signal charges transferred by the horizontal CCD 3. Here, the arrow M represents the direction in which the observed image focused on the imager moves. Reference characters A to D designate regions on which a part of the observed image is focused.
Now, it is assumed that a part of the observed image is focused on the region A. When a part of the observed image is supposed to be O.sub.1, signal charges in accordance with the observed image O.sub.1 are stored at the light-to-electricity conversion part 1. The stored signal charges are read out to the vertical CCD 2 by turning on the transfer gate 4 while the observed image O.sub.1 moves from the region A to the region B. The read out signal charges are transferred in the vertical CCD 2 in the vertical direction. The transfer speed then is equal to the velocity of the observed image.
Subsequently thereto, after the observed image O.sub.1 has reached the region B, signal charges corresponding to the observed image O.sub.1 are stored at the light-to-electricity conversion part 1 of the region B. The amount of the stored signal charges is equal to the signal charges stored at the light-to-electricity conversion part 1 of the region A. Then, the next observed image O.sub.2 is focused on the region A. Then signal charges stored at the light-to-electricity conversion part 1 of the region B are read out to the vertical CCD 2 from the light-to-electricity conversion part 1 by turning on the transfer gate becomes "ON" state while the observed image O.sub.1 moves to the region C from the region B after a predetermined storage time, and are added to the signal charges read out at the region A which are transferred to this part of the vertical CCD 2. In this way, the total signal charge amount corresponding to the observed image O.sub.1 is doubled. Accordingly, when this operation is repeated n times with n light-to-electricity conversion parts 1 in the vertical direction, the signal charge amount is multiplied n times and the shot noise becomes .sqroot.n times. The signal charges thus accumulated n times in the charge amount in the vertical CCD 2 are moved to the horizontal CCD 3 and transferred to the output part 5 to be output.
When the T.D.I. operation is repeated n times in this way, the signal-to-noise ratio in the vertical CCD 2 is improved by .sqroot.n times.
In the prior art one-dimensional solid-state imager carrying out T.D.I. operation, when n light-to-electricity conversion parts are used, n times the original signal charge is stored at the part of the vertical CCD relative to those which do not carry out T.D.I. operation. However, if the capacity of the vertical CCD is equal to that of the prior art one-dimensional solid-state imager, an overflow of charges arises in the vertical CCD and the observed image is not correctly recognized, and blooming results. If the capacitance of CCD is assumed to be in proportion to the area of the vertical CCD, this means that the area of the vertical CCD is increased. When a fundamental cell (hereinafter referred to as a pixel) is assumed to be constituted by the light-to-electricity conversion part, the transfer gate corresponding thereto, and the vertical CCD, the numerical aperture, which is a reference value of sensitivity in the general solid-state imager is defined as in the following. That is, the numerical aperture is a ratio of the area which is occupied by the light-to-electricity conversion part to the total pixel area. From this value, it is apparent that as the area other than the light-to-electricity conversion part becomes small inside the pixel, the numerical aperture is increased and the sensitivity is also increased. Accordingly, in the prior art device carrying out T.D.I. operation, when the area of the vertical CCD is increased to prevent blooming, the numerical aperture is reduced and, as a result, enhancement of sensitivity which is an object of the T.D.I. operation is prevented.